The invention is a control system for an electrostatic plotter or printer which permits the plotter or printer to operate at optimum performance despite the use of various media types and despite variations in media conductivity due to humidity changes.
Electrostatic plotters and printers are well-known in the art and operate by longitudinally feeding a sheet of printing media underneath a transverse line of small imaging electrodes in an imaging head. Individual electrostatic pixels of an image are formed on the media by applying electrical image writing pulses to corresponding ones of the small imaging electrodes at appropriate times at the media moves under the imaging head. The media then passes a toning station where liquid toner is applied to the media. The toner is attracted to the electrostatic image on the media and forms a permanant image thereon. For high resolution images with small pixels, the image writing pulses applied to the electrodes are very short in time, on the order to microseconds. The density or darkness of the image formed on the media is proportional to the amount of charge imparted to the media under control of the electrodes. This is because the greater the electrostatic charge at a given point, the more toner is attracted thereto.
Various electrostatic printing or copying techniques are known in the art. For example, U.S. Pat. No. 4,794,422 to Lehman et al. is directed to the overall control of a xerographic process machine. U.S. Pat. No. 3,954,333 to Goel shows controlling the image transfer roller in a xerographic process machine to optimize the transfer of image-bearing toner from the roller to the media. The roller is controlled as a function of resistivity changes caused by changes in ambient temperature. The media type is not considered. U.S. Pat. No. 4,460,270 to Watai et al. concerns controlling the intensity of an exposure lamp in a xerographic process machine as a function of sensed intensity compared to optimum intensity for the conditions. U.S. Pat. No. 4,354,758 to Futaki, like the patent to Watai et al., concerns controlling the intensity of an exposure lamp in a xerographic process machine. In the patent to Futaki, the lamp intensity is controlled as a function of sensed reflected light. U.S. Pat. No. 3,781,105 to Meagher is directed to controlling the parameters of a transfer member in a xerographic process machine as a function of sensed current variations. U.S. Pat. No. 4,431,302 to Weber discloses a corona discharge system in which a corona is employed to charge a medium and the speed of the medium is changed as a function of the sensed intensity of the corona field. U.S. Pat. No. 4,395,112 to Miyakawa et al. also concerns a xerographic process machine and is directed to sensing relative humidity to control a bias voltage employed to remove background fog from the image. No consideration of the media is made.
One problem with electrostatic printers and plotters is that the image formed on the media may have longitudinal stripes formed by longitudinal regions on the media of different images densities. This is one of the manifestations of non-uniformities in charge transfer from electrode to electrode along the electrostatic image head which lead to slightly different rates of charging across the plot. The stripes or "vertical banding" of dark and light vertical stripes are due to edge effects of multiplexed groups of nib electrodes. To achieve uniform charge transfer, all nib electrodes must transfer maximum charge to completely saturate the media. If maximum charge transfer is not achieved bands become visible.
The amount of image charge is determined by the media's electrical conductivity, which depends upon the type of media used in the electrostatic printer. Furthermore, the media conductivity changes with the ambient humidity in a manner which is different for different types of media. As a result, the performance of an electrostatic printer or plotter varies with the humidity and according to the type of media used. Thus, the same electrostatic plotter may produce perfect images one day and then--with either a reduction in humidity or a change of media type--may produce striped or defective images the next day.
Such problems have been addressed in a limited fashion by attempting to seal the environment inside the electrostatic printer/plotter and regulating the humidity therein with a built-in humidifier. This approach requires the user periodically to replenish the humidifier's water supply, a task which is easily forgotten until the image quality degrades noticeably. Moreover, such an approach only addresses change in humidity and fails to provide for the different conductivities of the various media used in electrostatic printers and plotters. That is, it is a "one size fits all" solution, the effectiveness of which depends on the particular media actually in use. Thus, while the humidity may remain constant, the user may still find that the image quality degrades as soon as the media in the printer is replaced by other media having a different conductivity.
As initial investigations by the inventor herein indicated, if one charts image charge versus on-time for nib electrodes in a typical electrostatic plotter such as those made by the assignee of this application under particular conditions, image voltage will increase continuously as on-time is increased from 2 to 30 .mu.sec. Beyond 18 .mu.sec, however, there is little change in image voltage with increased on-time. These same plotters typically employ a 25 .mu.sec on-time. The inventor's investigations lead to the conclusion, therefore, that the on-time could have been reduced to say 18 .mu.sec without effecting plot quality to any appreciable degree. This 28% reduction in on-time could be translated into an increase in plotter throughput. This reduction could not be made across the board, however, as under different conditions the 18 .mu.sec on-time might be insufficient. Thus, these plotters employ an on-time which is set for worst case conditions and suffer the loss of performance possible when conditions are less than worst case.
Thus, it will be appreciated that there is a need for an electrostatic plotters and printers which consistently provide images of the highest quality without noticeable defects (such as longitudinal stripes caused by density variations) despite changes in relative humidity and despite changes in media type or media conductivity.
Accordingly, it is an object of the invention to provide an electrostatic printer or plotter which consistently prints each pixel in an image with the maximum amount of charge at any relative humidity level (within a fairly large range) on any one of several types of dielectric media characterized by different media conductivities.
It is another object of the invention to provide a control system for an electrostatic printer or plotter which measures media conductivity and changes the image writing pulse width so as to compensate for changes in media conductivity.
It is yet another object of the invention to provide a control system for an electrostatic printer or plotter which determines the image writing pulse width from the type of media used and changes the pulse width in response to a change in media type.
It is still another object of the invention to provide a control system for an electrostatic printer or plotter which determines the type of media being used, measures media conductivity and changes the image writing pulse width in response to a change in media conductivity by different amounts for different media types, so as to compensate for changes in media conductivity while accomodating the different requirements of the different media types for different image writing pulse widths.
It is a yet further object of the invention to provide a control system for an electrostatic printer or plotter in accordance with any of the foregoing objects which measures humidity in lieu of media conductivity.
It is a still further object of the invention to provide a control system for an electrostatic printer or plotter in accordance with any of the foregoing objects which measures humidity in lieu of media conductivity and which can respond to either a media conductivity measurement or a relative humidity measurement to control the image writing pulse width.
It is a correlative object of the invention to provide a control system for an electrostatic printer or plotter in accordance with any of the objects above which changes the transport rate of the media and the pulse rate of the image writing pulse in proportion to any change in the image writing pulse width, so that the transport speed is increased to enhance throughput whenever the media conductivity is high, and so that the transport speed is decreased to maintain maximum image quality whenever the media conductivity is low.
These and other objects and benefits of the invention will become apparent in the following description of the invention when taken in conjunction with the accompanying drawings.